The composition and weathering of the continents over geologic time

Abstract

The composition of continental crust records the balance between construction by tectonics and destruction by physical and chemical erosion. Quantitative constraints on how igneous addition and chemical weathering have modified the continents' bulk composition are essential for understanding the evolution of geodynamics and climate. Using novel data analytic techniques we have extracted temporal trends in sediments' protolith composition and weathering intensity from the largest available compilation of sedimentary major element compositions: ∼15,000 samples from 4.0 Ga to the present. We find that the average Archean upper continental crust was silica-rich and had a similar compositional diversity to modern continents. This is consistent with an early Archean, or earlier, onset of plate tectonics. In the Archean, chemical weathering sequestered ∼25 % more CO 2 per mass eroded for the same weathering intensity than in subsequent time periods, consistent with carbon mass balance despite higher Archean outgassing rates and more limited continental exposure. Since 2.0 Ga, over long (>0.5 Gyr) timescales, crustal weathering intensity has remained relatively constant. On shorter timescales over the Phanerozoic, weathering intensity is correlated to global climate state, consistent with a weathering feedback acting in response to changes in CO 2 sources or sinks.

Igneous rock area and age in continental crust

“…However, the mean felsic-tomafic ratio of igneous rocks in Macrostrat columns is not markedly different in the Archean (Fig. S1 in the Supplemental Material), and major elemental data from sediments also indicate that Archean crust had a compositional diversity similar to that of modern continents (Lipp et al, 2021). Correcting DZ abundance for changes in the zircon saturation of magmas (Keller et al, 2017) does significantly increase DZ density estimates in the Archean (Fig.…”

Progress towards an improved Precambrian seawater 87Sr/86Sr curve

“…Plate tectonics is a defining characteristic of the modern Earth System that involves a globally linked system of lateral motion of rigid surface plates with lithosphere formed at mid-ocean ridges and consumed in subduction zones. When and whether modern style plate tectonics operated during the Archean remain topics of considerable debate and rigorous research (Brown et al, 2020;Cawood et al, 2018Cawood et al, , 2006Dhuime et al, 2015Dhuime et al, , 2012Ernst et al, 2016;Greber et al, 2017;Griffin et al, 2014;Harrison et al, 2008;Hawkesworth et al, 2017Hawkesworth et al, , 2010Hawkesworth et al, , 2020Hopkins et al, 2008;Keller and Harrison, 2020;Komiya et al, 1999;Kröner and Layer, 1992;Lipp et al, 2021;Moyen et al, 2006;Nutman et al, 2002;Smithies et al, 2007;Stern, 2018;M. Tang et al, 2016;Taylor and McLennan, 1985;Van Kranendonk, 2010;Van Kranendonk et al, 2007;Windley et al, 2021).…”

Earth system science applications of next-generation SEM-EDS automated mineral mapping

“…Superimposed on tectonically and climatically driven cyclical changes in weathering intensity (Frings, 2019) the step-wise greening of the continents via the evolution of microbial mats, fungi, lichen and ultimately land plants has long been thought to have driven a secular change to the continental weathering regime (Algeo et al, 2001;Kennedy et al, 2006;Lenton and Daines, 2017;McMahon and Davies, 2018). This is challenged by conventional bulk mineralogy and geochemistry that suggest that the composition of sedimentary sequences has remained largely unchanged since the emergence of the first continents (Tosca et al, 2010;Wei and Algeo, 2020b; Frontiers in Earth Science frontiersin.org 13 Lipp et al, 2021). A key limitation of bulk rock characterization approaches is their inability to differentiate sedimentary constituents that are products of weathering versus those arising from post-depositional alteration.…”